Air bubble flotation processes are widely used for particle separation and recovery in the minerals, coal, oil sands, and pulp and paper industries. However, conventional air bubble flotation systems are ill equipped to efficiently and economically separate and recover fine particles with diameters of <10 μm. This is due in part to the differences in the associated physiochemical and hydrodynamic conditions that exist in fine particle flotation systems. More specifically, fine particles have small masses and low inertia. As such, the fine particles tend to become entrained in the liquid streamlines that flow around the rising air bubbles, resulting in a low collision efficiency of the fine particles with the flotation air bubbles. Even after contacting the air bubbles, the fine particles may not have sufficient kinetic energy to rupture the intervening liquid films between the particle and the air bubble, which is required to achieve three-phase contact. The resultant retardation of three phase contact formation further contributes to a low attachment efficiency (Ralston, et al., 1984, Kent and Ralston, 1985).
To improve collision efficiency, the use of microbubbles as flotation carriers was developed and implemented (Yoon, 1993). However, fine particle flotation systems need an excessively large amount of small and stable air bubbles in order to provide sufficient bubble surface area flux for the required carrying capacity. Recently, hydrodynamic cavitation (Zhou, et al., 1997) incorporated into feed aeration (Xu, et al., 1996) was proposed for improving fine particle flotation, and resulted in the development of picobubble technology for fine coal flotation (Hart, 2001). As in microbubble flotation, the use of picobubbles (<100 μm) as carriers in fine particle flotation systems often results in a higher water recovery, and hence an increased mechanical entrainment of fine gangue particles. This results in a poor separation efficiency (grade of target particles).
Oil flotation (or liquid-liquid extraction) was proposed and showed great potential for improving fine particle recovery (Ralph, et al., 1968, Zambrana, et al., 1974). In this technology, oil droplets, instead of air bubbles, were used as carriers for hydrophobic particle flotation. Small, stable oil droplets can be produced by emulsification techniques, and their diameters can be varied from about 0.5 μm to about 50 μm (Ralston, et al., 1984; Kent and Ralston, 1985). Generating oil droplets in this size range is much more feasible than generating air bubbles of similar sizes. Moreover, since an oil droplet has a higher inertia than an air bubble under the same physical and hydrodynamic conditions, the collision efficiency of the fine particles with the oil droplets tends to be higher than with similarly sized air bubbles. Additionally, fine particles are more easily collected at an oil-water interface than at an air-water interface (Fuerstenau, 1980). This is due mainly to stronger spreading potency enhanced by a stronger long-range attractive molecular force between oil and solid than between air and solid interfaces. The use of oil droplets as flotation carriers can also minimize the water recovery, as the spreading nature of oil on solids displaces the otherwise retained process water which collects around air bubbles and the solids.
The feasibility of oil droplet flotation has been demonstrated at laboratory scale for recovering particles less than 10 μm (Kusaka, et al., 1994, 1997). However, direct application of oil droplet flotation to fine particle flotation at a commercial scale was considered uneconomical, due to the large amount of oil needed for operation. This is one of the reasons why the earliest flotation processes, known as Haynes, Everson and Cattermole processes (Fuerstenau, 1980), which used bulk oil as the carrier for hydrophobic particle flotation was not applied on a commercial scale. As in conventional flotation with air bubbles as carriers, in flotation systems with oil droplets as carriers, the collector was added to the aqueous phase. Thus, a large quantity of the collector was required for this process. Moreover, the presence of collector on mineral surfaces and at oil-water interfaces retarded the attachment process of the particle with the oil droplets due to a lack of direct contact between the two.
More recently, employed in coal cleaning are oily droplet carriers that have oil-soluble collectors dissolved therein. This is referred to as emulsion flotation and is accomplished by emulsifying collector-containing in the oil phase (e.g. kerosene) with the addition of promoters in the oily collector phase to enhance the dispersion of oily collectors (Laskowski, 1992). Emulsification of an oily collector increases the number of oil droplet carriers, and therefore aids in the flotation kinetics of the system. The addition of promoters also reduces the oil/water interfacial tension, leading to improved spreading of collector-containing oil droplets on target particle surfaces. The objective of using oil in this case is to enhance collector distribution in the pulp. The increased flotation recovery by the emulsified oil droplets has been reported (Laskowski, 1992). However, since a large quantity of oil is needed in this process to form the oil droplets, a corresponding large amount of collector is required for dissolution into the oil droplet. Unfortunately, this encourages the transfer of collector from the oil droplet to the aqueous medium. As described above, in addition to a large amount of oil and collector required for this approach, the presence of collector on mineral surfaces and at oil-water interfaces of the oil droplets retards the attachment process of the mineral with the oil droplets due to a lack of direct contact between the two.
A technique of using air to promote oil agglomeration was developed (Shen and Wheelock, 2000, 2001) for fine coal cleaning, in an attempt to minimize the overall oil consumption and to enhance the coal cleaning. In this process, a small amount of air (entrained, added or supersaturated) was introduced into the coal slurry with added hydrocarbon oil under high shear conditions. Tiny air bubbles were formed by gas nucleation and cavitation in the slurry. Based on the original model suggested by Taggart (1927), an oil film would spread on the bubbles, and as such the oil-coated bubbles could act as carriers for collecting hydrophobic coal particles to form compact gas/oil/coal agglomerates. Compared with the cases without air addition, the recovery and separation efficiency of fine coal was increased, while the amount of oil required for achieving the same efficiency was reduced, especially for moderately hydrophobic coal flotation. The possible application of oily bubbles to enhancing de-inking efficiency has been demonstrated recently (Finch, et al., 2001). However, in this process all of the process aids were added to the aqueous phase resulting in the same shortcomings noted above for the other two processes.
Accordingly, there remains a need for a particle flotation system that economically and efficiently achieves a high percentage of target particle separation and recovery.